Electrolytic cells are known for the extraction of valuable materials by deposit upon a removable electrode, in particular the deposit of metals upon a removable cathode. Electrolytic silver recovery from used photographic solutions is a common way to extend the life of such solutions. In an apparatus for the electrolytic removal of silver, for example from used photographic fixer, the cell includes a housing, having a contact surface for making contact with a removable cathode when positioned in the housing, to supply electrolysing electrical power thereto. The contact surface is positioned above the liquid level in the cell. The cell is used by supplying electrical power to the cathode at an electrolysing potential via the contact surface. In use, silver from the silver containing solution is deposited on the face of the cathode which is directed towards the anode. After the cell is operated for some time, the cathode is removed from the cell and replaced. U.S. Pat. No. 5,378,340 (Michiels et al. assigned to Agfa-Gevaert NV) issued Jan. 3, 1995 describes an example of such an apparatus.
For optimum efficiency, the electrolysis process is controlled, in particular by seeking to control the potential on the cathode. This is achieved by controlling the potential applied to the contact surface.
However, in practice, the electrical contact between the contact surface and the cathode is not perfect. There tends to be a resistance between the contact surface and the cathode, which resistance is unknown and may well vary as electrolysis continues.
The high current which passes through the cell during electrolysis results in a significant potential drop across the junction between the contact surface and the cathode. As a result, the potential on the cathode is not necessarily the same as that applied to the contact surface. While the control regime applied to a given cell can be modified to account for this effect, such modification would not account for changes which might occur during electrolysis.